Strategies to Reduce Urban Pollution Effects on Solar Panels: A Review

Abstract

Urban soiling, consisting of dust, industrial byproducts, and other pollutants, presents a significant risk to the effectiveness and safety of solar energy systems. To achieve the goal of net zero, having renewable energy systems such as solar panels in urban environments can help. This review will examine the composition and variety of urban soiling and evaluate its impact on PV installation. The study will analyze the efficiency loss attributable to soiling, focusing on its impact on small-scale installations such as rooftops, building integrated photovoltaics (BIPVs), and large-scale urban solar installations. Furthermore, this study will also investigate various developing technologies and strategies to reduce the effects of urban soiling. This encompasses the examination of automated cleaning systems and robotic maintenance, with a specific focus on their potential effectiveness. This review aims to underline the importance of addressing urban soiling within the framework of sustainable urban development and the expansion of solar energy, with further research into the development of soiling mitigation technologies. Finally, soil management and further research gaps will be discussed.

1. Introduction

Recently, the importance of reduction in soiling on solar panels has gained more attention, particularly in the context of its implications for achieving net-zero carbon goals [1]. As urban areas continue to expand, the buildup of pollutants and particle matter on surfaces such as solar panels become a significant concern, directly influencing the efficiency of solar energy systems and thereby impacting global carbon reduction efforts. This paper aims to review past studies about urban soiling with PV technologies, and the future work of potential mitigation strategies. Several objectives will be considered as the basic background introduction to the growing importance of solar energy and PV systems in urban environments. This study examines the general ideas about urban soiling, the potential reasons for its cause, and the effects of urban soiling on solar power performance, including degradation, transmittance, and efficiency [2]. This study also discusses where PV installation, as building integrated photovoltaics (BIPVs) and possible rooftop solar systems in the city, can improve the potential power production from renewable resources. Figure 1 indicates the potential idea that air pollution leads to soiling in the urban environment.

Urban soiling considerations can be varied by region; in this paper, the potential regions where urban soiling is of more significance will be discussed [3]. Asia has a specific soiling composition due to its large population density and rapid industrialization, particularly in countries like China and India. These regions often experience heavy pollution from industrial emissions and vehicle traffic, leading to a higher concentration of soot and fine particulate matter accumulating on solar panels. The United States, with its diversified industrial and climatic geography, also experiences a variety of soiling compositions. In the United States, urban areas with heavy traffic and industrial activities have a different soiling profile compared to less industrialized locations. Europe, which has a mix of urban and rural landscapes, presents another variation in soiling composition, influenced by both natural dust and urban pollutants. Figure 2a indicates the potential number of installed PV systems around the world from 2012 to 2023 [4], and Figure 2b presents the data of installed PV systems for a few stand-out metropolitan locations around the world in 2023, showing China and the USA have a greater PV installation number compared to the rest of the world.

Case studies of solar power plants in urban areas, where the results may differ due to lower transportation-related pollution, were excluded in this instance due to the mention of large-scale PV plants. BIPVs, as a type of technology which is related to urban soiling, especially in urban areas, has been highly discussed and reviews will be included in this paper to provide a better basic understanding. To increase the possibility of less loss and greater efficiency, cleaning is essential in the urban environment as a part of the impact.

2. Characterization of Urban Soiling

Urban soiling, a term that refers to the accumulation of pollutants on built surfaces in city environments, is recognized for its increasing impact on urban infrastructure and public health.

2.1. Definitions and Properties

Urban refers to characteristics that are connected to a town or city. Thus, soils found in urban environments are those that have been impacted or changed by human activities, and the term does not apply to soils that are used for intensive agricultural activity. This distinction suggests certain baselines for an existing population, population density, variety of man-made structures, and human activities that impact the native soil ecology [5]. Therefore, urban soiling, through research, a significant factor impacting the efficiency of solar photovoltaic systems, particularly in densely populated areas, refers to the accumulation of diverse pollutants on the surface of solar panels [6].

Considering the characteristics of urban soiling, which contains a range of physical and chemical properties [7], these characteristics have a substantial influence on the operational performance and efficiency of photovoltaic (PV) systems, which will be discussed in detail in the section on impact. The physical qualities, such as adhesion and cohesion, particle size distribution, optical properties, and surface roughness will also be considered [8].

The strength of the bond between soiling particles and the solar panel surface, along with the cohesive forces within the soiling layer itself, determine the tenacity of the soiling and its ease of removal [9]. Soiling particles can vary in size, from microscopic dust to larger fragments. The size distribution influences the shading of solar cells and the scattering of light, which can impact the amount of sunlight reaching the photovoltaic material [10]. The color and transparency of the soiling layer directly affect its optical density and therefore its effect on light transmittance [11]. The accumulation of soiling can also result in a change in the roughness of the solar panel surface, leading to light reflection and further reduction in effective light absorption [12]. Some urban soiling may exhibit hygroscopic properties, meaning it can absorb moisture from the atmosphere. This can also impact the adhesion and removal of the soiling from solar panels [13]. Urban soiling is chemically varied, considering its organic compounds, inorganic salts, acids, and heavy metals [14]. Certain chemical components of soiling can react with the materials in solar panels, potentially causing corrosion or other possible forms of damage such as degradation [15]. The hygroscopicity of some soiling can contribute to moisture accumulation, which may lead to further disruption of light transmission and potentially inducing localized corrosion [16]. The authors of [17] also indicated both the physical and chemical properties of soil, with analysis of its various moisture content, mean particle diameter, coarse fraction, carbon and nitrogen concentrations, and saturated soil permeability, as Figure 3 presents.

2.2. Origins and Contributors to Urban Soiling Phenomena

As mentioned, industrial, transportation, and construction contributions are the main components of the sources of urban soiling [18]. Various pollutants which originate from different urban activities include dust, vehicle emissions, industrial byproducts, and construction debris. The authors of [19] included the concentration of most chemical properties, which are mentioned to be considerably higher in parks, streets, and old residential areas compared to both old and new mulch and new residential areas. Based on the research, Figure 4 presents real pictures of solar panels in various settings, and

Table 1. Potential sources of urban soiling.

Source Category	Specific Sources	Description
Transportation emissions	Brake and tire wear, non-exhaust emissions	Emissions from vehicles, including unburned hydrocarbons, metal particles, and soot from engine wear and combustion.
Industrial activities	Factories, manufacturing locations	Emissions from industrial processes, including particulates and gases.
Construction sites	Building materials, dust	Dust and debris from construction and demolition activities.
Energy production	Power plants	Emissions from energy generation, particularly the combustion of fossil fuels, sulfur dioxide emission, and nitrogen oxides into the atmosphere.
Urban infrastructure	Road, bridge, and building wear	Particulates generated from the wear and tear of urban infrastructure.
Biological materials	Bird droppings	Organic soiling from birds, and possible microbial growth.

presents potential information about urban soiling, including the specific sources and their descriptions [12,20,21,22,23,24].

3. Implications of Soiling on Urban Environments

3.1. Urban Soiling and Challenges Posed by Degraded Soil

The deterioration of soil in urban contexts has become a significant problem in the field of environmental studies, leading to the difficulty of the maintenance and cleaning of solar energy systems. This degradation is caused by a combination of human activities and natural processes [24], where the deterioration of soil in urban environments is characterized by multiple impacts that have been mentioned in the previous section, including physical, chemical, and possible biological modifications. As for sandy soil, it is not too difficult to remove with water or mechanical cleaning; however, clay particles tend to adhere to the surface of the PV panel, which requires a more intensive cleaning process.

As researched, the effectiveness of solar panels is significantly affected by soil deterioration, which is mostly caused by a combination of environmental and human-related causes [28]. Dust and particle matter in the environment, commonly found in dry or dusty areas, significantly contribute to the buildup of soil on the surfaces of solar panels. Additionally, industrial residues such as soot may require a chemical cleaning process. Regular cleaning and maintenance are crucial, otherwise, permanent damage can happen, which leads to reducing the efficiency and lifespan of the solar panels [29]. In [30], the main soiling factors were rainwater residues, with a 2% optical loss, as well as bird droppings, leading to a 15.5% power loss. In [31], which expressed an experiment with different types of industrial dust deposition on solar panels under three different solar irradiances, dust particle sizes ranging from 20 to 45 µm were used. The result of dust on the solar module caused a temperature difference between the clean and dirty modules, ranging from 1 °C to 9 °C. It also indicated that dust deposition caused a decrease in the current output (I_SC) and power output (P_max) of the photovoltaic module. To give a better idea of the different causes of soil degradation under urban conditions, Figure 5 is presented [32].

3.2. Impact of Pollution on Solar Panel Performance and Power Generation

In this section, the impact of urban soiling will be discussed, including the potential impact on PV panel performance, and specifically, the impact on power generation. The accumulation of pollutants on PV panels will reduce efficiency and long-term performance. There are several concerns, one of which is heat dissipation while the dust and pollutants accumulate on the surface of PV panels. They act as a layer which traps heat and increases the operating temperature of the panels, which can lead to thermal stress and reduce the lifespan of the panel [28]. Additionally, the capability of light absorption can be affected by soiling. Dust can block sunlight, which can reduce the amount of light that reaches the PV panels. This can result in a decrease in the energy conversion efficiency of panels. When considering the urban environment, soils act as a source of air pollutants, particles, and gases [33]. The authors of [24] indicated that dust storms can be caused by both natural and human activities and can directly impact air quality by releasing dust particles into the atmosphere. They also indicated that the tilt angle of the PV module and the use of an anti-reflective coating on the glass cover were found to reduce both the accumulation of dust deposition and the loss in transmittance.

In [34], the authors conducted an investigation of the effects of dust fouling on the transmittance of glass covers for PV modules in a harsh environment. The study quantified the electrical energy generated by PV modules while subjected to environmental conditions, with modules tilted at a latitude of 26°. Two test stands were used to measure the overall transmittance of the glass cover.

Table 2. Recorded dust deposition and transmittance reduction with varied tilt angles.

Tilt Angle (deg.)	$\mathbf{Dust} \mathbf{Deposition} (\mathbf{g}/{\mathbf{m}}^2$)	Transmittance Reduction
0	6.5	0.21
15	5	0.19
30	4.9	0.16
45	4.5	0.15
60	3.1	0.13
75	2.1	0.11
90	0.9	0.04
0	6.5	0.21
15	5	0.19
30	4.9	0.16
45	4.5	0.15
60	3.1	0.13
75	2.1	0.11

indicates the glass transmittance and dust deposition after 45 days of exposure.

The power output of a photovoltaic module is strongly affected by the level of irradiance that reaches the solar cells. Several variables contribute to the optimal output or maximum efficiency of a solar module [35].

Rather than just measuring the power output to indicate the impact, in the PVCastSOIL project [36], the influence of soiling losses was measured by using an electrical setup with PV modules and a soiling test bench with glass coupons. Other than the hemispherical transmittance measurements, standard meteorological measurements were also taken, such as the ambient temperature, relative humidity, wind speed and direction, and rainfall. These measurements were obtained on the rooftop and in a water tower located nearby. The soiling loss was estimated using a performance index metric (PIIsc) for both the dirty and reference modules [26]. The performance index was calculated by considering the temperature-corrected short-circuit current and the soiling loss was defined as the difference between the performance index of the soiled module and the reference module. Modules with lower tilt angles and higher incidence angles had greater soiling losses.

Most research investigates urban soiling with a specific particle, but [2] presents the impact of different types of pollutants on the performance of photovoltaic (PV) panels. In Figure 6, the concept of soiling deposition on rooftops is presented. The study [2] focuses on the influence of dust buildup on the efficiency of PV systems and the specific types of pollutants found in different geographical locations. The study found that dust pollution affected the current and voltage of PV cells by blocking light. The impact of red soil accumulation on PV performance was the most notable, followed by limestone and ash. A deposition of 0.35 g/m² of red soil caused a drop in energy production of almost 7.5%, whereas the same deposit density of limestone caused a drop of almost 4%. Ash had the largest effect on PV performance compared to other pollutants, given the statement that the reduction in output efficiency of PV modules increased directly proportional to dust deposition [2].

The impact of photovoltaic (PV) systems on urban environments, focusing on the efficiency and power production of PV systems in densely populated areas, is discussed in [37]. By providing qualitative sensitivity analysis, the maximum power output of PV modules is affected by changes in cell temperature, specifically, for different module technologies. Factors such as elevated air temperatures, air pollution, partial shading, and soiling have a substantial influence on the efficiency and power generation of PV panels in urban settings.

Other than knowing the impact on PV panels, given the broader aspects of the impact of urban soiling on the environment and future development, its impact on solar PV plants are reviewed. In [38], the author presents experimental data related to soiling losses from a region (Rome) in Italy which is suburban. These data provide a deeper comprehension of soiling losses associated with PV systems mounted on urban infrastructure. The study also assesses and refines the current soiling estimation models, which are primarily designed for large-scale utility purposes. The research is consolidated into three distinct sections, encompassing the soiling loss formulation, the integration of soiling models, and the incorporation of models that harness satellite-derived data, accumulating a broad spectrum of variables. The results indicate pronounced soiling during summer months when less rainfall happens.

$$S_L=1-{\displaystyle \frac{{ISC}_{soil}}{{ISC}_{ref}}}$$

(1)

$$S_L=0.3437\mathrm{e}\mathrm{r}\mathrm{f}\left(0.17w^{0.8473}\right)$$

(2)

$$w=\sum (v_{10-2.5}\left[{PM}_{10-2.5}\right]+v_{2.5}\left[{PM}_{2.5}\right])tcos\theta$$

(3)

During this period, soiling tends to increase by approximately 0.07% daily, implying an accumulation of over 2% in less than 30 days by using the potential Equations (1)–(3) from [38]. The variables ${ISC}_{soil;}$ and ${ISC}_{ref}$ represent the short-circuit currents of the soiled and reference clean cells; $S_L$ stands for soiling loss; $w$ represents the cumulative sum of the daily particulate matter that has accumulated on the PV surface since the last natural or artificial cleaning; $t$ is the conversion factor from seconds to daily units; and $\theta$ represents the tilt angle [38].

However, there are certain constraints as the experimental studies, similar to the current one, are important in comprehending the extent of soiling losses in large magnitude settings, especially in areas that have been under-researched which may experience a substantial augmentation in integrated PV capacity. It is necessary to validate soiling estimation models, which have the potential to be instrumental in monitoring soiling losses, especially in BIPVs, where sensor implementation might be challenging or infeasible. Nevertheless, these models, predominantly constructed for non-urban, large-scale utility installations, demand further validation and calibration. Dust deposition is a significant challenge for photovoltaic (PV) power systems due to its negative impact on their efficiency and economic viability [39]. The decrease in power production caused by soiling diminishes the material efficiency of PV modules, hence adversely impacting the sustainability of this technology. Furthermore, soiling not only diminishes the ability of glass to transmit light but can also result in lasting deterioration by promoting the development of localized high temperatures and abrasion [15].

While soiling losses approximated 0.2% per day using assessments of short-circuit current evaluations, it was demonstrated that the actual production owed by soiling is based on the fraction of DC/AC oversizing [40]. It is imperative to note that soiling-induced losses are predominantly contingent on geographical factors; such losses can exhibit variation due to elements including climatic conditions, meteorological factors, seasonal transitions, distance to industrial operations, etc.

In [41], the authors provided insights into the effects of air pollution and soiling, and the benefits of their elimination, on solar PV systems worldwide. The study found that highly polluted areas, such as the Northern China Plain and Indo-Gangetic Plain, experience significant reductions in PV capacity factors (CFs) due to atmospheric aerosols. Aerosols have a more significant influence on tracking PV modules compared to fixed-tilt modules. The Middle East shows the highest PV CF losses, while Oceania has the lowest reduction. The study also predicted the future PV capacity and reductions in CFs for several countries. An analysis was conducted on the influence of air pollution on solar PV power generation in urban areas, with urban air pollution negatively affecting PV potential. The study highlighted the impact of the COVID-19 lockdown measures on the solar PVs industry, leading to reduced air pollution and increased surface solar radiation, and explores various soiling mitigation approaches, including natural cleaning, anti-soiling coatings, mechanical cleaning, and electrodynamic dust shields [32]. For researching the impact of soiling on solar power plants, Saudi Arabia in the Middle East is one of the most popular countries [37].

The results of the microhardness of mud are affected by the presence of closely packed particles and voids between larger particles. The amount of work needed to remove the mud from the glass surface due to friction is significantly less than the work necessary for the combination of cohesion and adhesion [42]. The coefficient of friction is higher for the glass surface after the mud has been removed compared to the glass surface in its original condition. The mud formed from the dust has a significant impact on the qualities of the glass, such as its absorption, transmittance, microhardness, and surface texture characteristics. The adhesion and cohesion force necessary to eliminate the dirt from the glass is higher than the frictional forces exerted on the glass surface [43]. In [44], a method is proposed that uses thermography to analyze the temperature distribution of PV modules to discover mismatch faults. The study classifies faults into three categories, with minor, medium, and heavy faults, and investigates them analytically and experimentally. In [45], the fault diagnoses in the experiment involved testing a PV module under various load situations, such as the MPP load, heavy load, light load, and shadow conditions.

$$V_{eq}=V_{string}-\left(n-1\right)V_s+(m-n)V_d$$

(4)

$${ V}_s\approx {\displaystyle \frac{V_{MPP,stc}-V_{OC,stc}}{I_{MPP,stc}}}{I}_{string}+V_{OC,stc}$$

(5)

$$n=\mathrm{1,2}\dots m, for (m-1)\alpha V_{OC}<V_{string}<=m\alpha V_{OC}$$

(6)

Equations (4)–(6) [46] suggest that the photovoltaic array can be defined with $V_{eq}$ as the equivalent value of voltage, and $V_{string}$ and $I_{string}$ noting the voltage and current output of the PV string, respectively. The term $V_d$ signifies a constant value, while $V_s$ delineates a coefficient within the linear source framework, as delineated in (5). The variable $m$ is the sum of the photovoltaic modules constituting the string, and $n$ is expressed in (6). The notation $V_{OC}$ specifies the open-circuit voltage inherent to the PV modules. Moreover, $V_{MPP,stc}$ and $I_{MPP,stc}$ are the voltage and current, respectively, at the Maximum Power Point (MPP) under Standard Test Conditions (STCs). The coefficient $\alpha$ is an adjustable parameter within the range of 0.8 to 0.97.

The authors of [47] support the advancement of the growing research field in improving the treatment and management of urban soil and the supply of ecosystem services. The analysis was initially conducted on all categories of the literature, with a more thorough analysis performed on the quantified publications to identify the studied ESs, their co-occurrence, and the most recorded soil depths. The results of the review found that urban agriculture has a lack of research on the raw materials from urban soil and cultural services. The interrelation between the different ESs studied is restricted, with the majority of research concentrating on soil depths between 0 and 20 cm. They provide the idea of having future research focus on urban agriculture, cultural services, global representation, the interconnection between researchers and policy, and the drivers of change in urban ecosystems.

The objective of [42] is to assess the influence of aerosols on photovoltaic (PV) generation by studying the worldwide effects of atmospheric aerosols and soiling at regional and subnational scales. In the paper [42], the consideration of exploring the integration of solar PV performance models with long-term satellite-observation-constrained data on surface irradiance, aerosol deposition, and precipitation rates was taken. The aim was to provide a comprehensive understanding of the worldwide effects of particulate matter (PM) on PV generation and to accurately assess the impact of particulate matter (PM) on photovoltaic (PV) generation. It is important to account for both the attenuation produced by PM in the atmosphere and the deposition of PM on PV panels. Additionally, the analysis should incorporate the effects of precipitation in removing soiling induced by PM and the advantages of cleaning the panels. In this paper, the following examination was taken of the global distribution of solar resources and photovoltaic electricity generation, and it investigated how they are affected by the influence of PM.

Further research with other methods was to determine the extent of soiling losses in PV power plants. This can be achieved by measuring the current–voltage (I–V) curves at both the module and string levels of the arrays [48]. The AC/DC disconnect switches were turned off and the fuses were removed from the combiner box. The I–V curves of individual modules were obtained by disconnecting the string to which the modules were connected and measuring the curves at the terminals of the modules. The results were that I-axis tracker-based modules in rural areas had a greater level of soiling loss, given that the small rainfall, measuring around 0.04 inches, was only 61% as effective as hand cleaning for the horizontal tilt modules.

$$\lambda ={\displaystyle \frac{N_d}{N_P}}100\%$$

(7)

In Equation (7), for the dust deposition rate, $\lambda$, and $N_d$ are the number of dust particles deposited on the solar panel, and $N_P$ is the total number of dust particles in the airflow [40]. Subsequently, the rates at which dust settles on the PV panel can be calculated for varying particle sizes and tilt angles of the panel.

To provide an review of the key finding across the studies, a research summary is presented in

Table 3. Research summary.

Study/Project	Focus Area	Key Findings
PVCastSOIL [29]	Soiling losses on solar panels	Utilized a soiling test bench and glass coupons for long-term soiling characterization. Found variations in soiling impact based on tilt angle and environmental conditions.
Study on dust impact [2]	Impact of different types of urban soiling	Explored the differential effects of various pollutants (red soil, limestone, and ash) on the efficiency of PV systems.
Study on urban soiling impact [31]	Urban soiling and PV systems	Highlighted the need for research on urban agriculture and cultural services in the context of urban soil ecosystem services.
Global impact of aerosols [32]	Aerosols and PV generation	Investigated the worldwide effects of atmospheric aerosols and soiling at local and subnational scales. Considered the reduction in intensity caused by air PM and the deposition of PM on panels.
Method for measuring soiling losses [33]	Soiling losses in PV power plants	Methodology involved measuring I-V curves at both the module and string levels. Found higher soiling losses in I-axis trackers in rural surroundings and the varying effectiveness of cleaning methods.
Study on air pollution and soiling [35]	Air pollution, soiling, and PV systems	Analyzed the effect of air pollution and soiling on solar PV systems worldwide. Examined the reduction in PV capacity factors due to atmospheric aerosols in different regions.

.

3.3. Analysis of Soiling Accumulation Rates in Urban Settings

Understanding the variability in urban soiling composition and the factors influencing its accumulation is vital for optimizing the maintenance and efficiency of solar PV installations in different urban areas. This knowledge enables the development of tailored cleaning schedules and technologies, ensuring maximum energy output from these renewable sources. The composition of this soiling is heterogeneous and significantly changes across different urban locations due to varied environmental and industrial influences. Several variables determine the pace at which these contaminants collect on solar panels. These include precipitation such as rainfall [49]. Regions experiencing regular rainfall may have reduced buildup since the precipitation effectively removes a substantial amount of dirt and grime. In contrast, areas with very little precipitation might have greater rates of dirt buildup. Moreover, the inclination at which solar panels are mounted has a substantial impact on the accumulation of dirt. Panel inclinations with a higher angle are less susceptible to the accumulation of pollutants due to the assistance of gravity in the natural elimination of particles. Urban rooftop installations with flatter panels are prone to retaining pollutants, which require more regular cleaning.

The authors of [50] assessed the effect of soiling on heliostats in a real solar field located in Madrid, Spain. They focused on monitoring reflectance, identifying seasonal phenomena such as Saharan desert dust, and characterizing particles using scanning electron microscopy. The impact of the tilt angle on reflectance loss was assessed by measuring reflectance at five distinct points in each heliostat. As it is an experimental paper, the results showed that the soiling ratio varied throughout the year, with seasonal patterns influenced by atmospheric particle concentration and precipitation. In this paper, the average soiling ratio between June and August 2020 was 10.3%, while from September to December 2020, it was around 3.11% due to higher rainfall frequency, which is presented in Figure 7. It can be stated that the soiling rates were higher during the late spring and summer, and lower towards autumn and winter. Other than the effect of rainfall frequency, the long-range transportation of Saharan dust during winter and additional organic matter accumulation led to higher soiling ratios. A soiling model was developed to optimize the assessment of soiling in the solar field, with linear equations chosen for the model [10].

4. Integration of Photovoltaics in Urban Infrastructure

In this section, the potential urban installation of PV systems will be discussed.

4.1. Focus on Building Integrated Photovoltaic (BIPV) Systems

Real-world examples of PV installations in urban settings include, e.g., rooftop solar panels (the most common usage and the most difficult one to consider regarding cleaning as it is not economically efficient), building integrated photovoltaics (BIPV), and vehicle solar panels. The process of incorporating solar modules into the building envelope, such as the facade or roof, is known as building integrated photovoltaics, or BIPVs [51]. BIPV systems may save money on materials and electricity, lessen the dependence of the building on fossil fuels, and enhance the architectural structural appeal by acting as both a power generator and a building envelope material. Air pollution has a significant impact on the performance of BIPVs, especially in urban areas with high levels of particulate matter. In highly polluted cities, the BIPV systems can experience a significant reduction in power output due to the accumulation of particulate matter (PM 2.5 and P.M 10). Additionally, gaseous pollutants such as Nox and Sox can lead to the formation of acidic deposits on the surface of the panel, and the urban heat island effect can lead to an increase in operating temperatures of BIPV systems.

Organic photovoltaic (OPV) cells are essential in the semi-transparent solar cell group, given their potential applications in BIPVs. These cells enable light absorption, which can effectively enhance power conversion. In research, the molecule orientation in OPVs influences charge transfer and stability [52]. New developments can improve the durability of Y6-based OPVs by mitigating thermal degradation challenges. Although these have several advantages compared to the other OPVs, they still have stability issues [53]. However, having these integrated into BIPVs can improve energy efficiency by maximizing transparency and performance reliability.

The paper [54] provides experimental results from various studies on the performance of semitransparent photovoltaic (STPV) modules. The studies evaluated the solar heat gain coefficient (SHGC) and optical properties of different STPV modules under various conditions. It also analyzed the daylighting and visual comfort provided by STPV modules, considering factors such as the transmittance level and glare probability. The color rendering index (CRI) and the impact of color and texture on energy performance were also discussed. Conversely, [55] reviews the applications of building integrated photovoltaic (BIPV) and building integrated photovoltaic/thermal (BIPV/T) systems in terms of their energy output, nominal power, efficiency, type, and performance assessment methods. This paper addresses the shortage of highly qualified workers for the installation and maintenance of BIPVs. It also discusses the PVTRIN project, which is funded by the European Commission. Additionally, the paper considered and discussed the use of photovoltaic glass in PV Curtain Wall applications, which can optimize the performance of the building envelope and enable the generation of energy on-site. The primary finding derived from the outcomes of this paper was that the fundamental BIPV technology and its applications have been thoroughly examined in prior research. It also stated that c-Si BIPV systems might potentially achieve a lower levelized cost of electricity (LCOE) compared to rack-mounted c-Si PVs, provided that their installation costs are at least 5% lower.

The study [56] identifies the barriers and difficulties in achieving widespread adoption of solar PV and BIPV systems in India and provides policy recommendations to overcome them [56]. It also explores the status of BIPV systems, including the global market growth and the pioneers in implementing BIPV technology in India. The article further examines the application of BIPV systems, including their use as an energy supply technology, demand management technology, backup power, and their architectural elements. The selection criteria for BIPV systems are discussed, considering elements such as location, improvements in BIPV technology, architectural design, and economic feasibility. An analysis is conducted on the barriers and policy challenges faced in the development of BIPVs in India, including the high initial investment costs, lack of technical experts, inadequate training, and conflicting policies. The results of the article emphasize the high solar irradiation potential in India and the significant increase in installed solar PV capacity due to government policies.

The impact of urban climate on the performance of BIPVs is presented in [57]. Factors such as solar radiation reduction, changes in solar spectral content, increases in urban air temperature, and lower wind speed in urban areas are taken into account. The article describes three models (Model A, Model B, and Model C) which are used to assess the performance of photovoltaic modules in urban environments. Model A calculates the monthly average electrical output of a PV array using characteristics such as temperature, solar radiation, and clearness index. Model B predicts the hourly electrical output of a PV module without considering the solar spectrum. Model C takes into account the spectrum response of solar irradiation on the PV module. This study used three different models to examine the influence of urban climate on PV module output [57]. Monocrystalline silicon modules are utilized on open racks in both urban and rural locations. The decrease in solar radiation in urban areas is found to be the primary cause of the decrease in PV module output. Given the found results of this study, the conversion efficiency of PV systems is better in urban areas compared to rural areas due to lower module temperature. It also emphasizes the need to consider the spectral response of PVs in urban environments to accurately estimate the DC power output. The vertical solar cells and rooftop solar panels in BIPVs and BAPVs are presented in Figure 8, for the potential of optimizing solar energy in urban environments.

The paper [61] presents a design tool that can determine the optimal tilt angle for various building surfaces (horizontal, vertical, and inclined), and evaluate the economic and technical elements of the systems [61]. This paper includes a technical section, which involves selecting suitable surfaces based on yearly solar radiation, placing panels and optimizing the tilt angle considering direct and diffuse irradiation, shading situations, and the CO₂ emission index. The economic section involves evaluating the financial viability of the BIPVs and determining the order of installation of surfaces based on economic indicators. The results of the study show that the proposed tool can provide a comprehensive review of the techno-economic feasibility of BIPV systems on different building surfaces and can help decision-makers identify the most appropriate surfaces for BIPV implementation in urban areas.

Research in only BIPVs is common, while the study [62] focuses on comparing the environmental and economic benefits of two photovoltaic (PV) systems: a building-applied PV (BAPV) system and a building-integrated PV (BIPV) system [62]. Automated data monitoring systems were implemented to collect data on the ambient temperature of photovoltaic (PV) systems, solar radiation levels, and power output. The weather station for both systems is situated close to the PV systems. The BAPV system is the first grid-connected roof-mounted BAPV system in China. The BIPV system employs polycrystalline silicon photovoltaic modules mounted on the roof to generate power for the building and decrease the amount of heat inside. Based on the results of this paper, the BIPV system offers superior environmental advantages compared to the BAPV system, with higher reductions in emissions and greater environmental economic benefits.

4.2. A Focus on Solar Panels in Housing and Industry Installation

Solar panels can be installed on rooftops either with stands with mounting systems that allow for angle adjustment, or without stands and mounted directly onto the roof. By considering BIPV technology and its potential usage, different types of solar panel installations under household and industry conditions should be considered.

Power output profiles may be adjusted to correspond better with household power consumption profiles by positioning solar panels at varying inclinations and azimuth angles [63]. However, solar panels with stands for rooftop households are rare, from the research. In [64], there is a comparison of the rooftop solar panels in households, which are at fixed angles, and those in some rural installations, where solar tracking panels are increasingly used for increasing efficiency. The authors of [65] considered a newly implemented rooftop solar PV system in India. A novel 248 kW PV system design was created to enhance the efficiency of the current PV installation situated on the rooftop of the building. The proposed design was evaluated using the PVsyst v6.70 software, which operates based on precise plant specifications and real-time data collected from the current plant. By using this technology, the solar panels can face different angles in different seasons for maximum radiation. However, the limitation of this study is the lack of real experiments that can be conducted, as in reality, it is rare to install solar panels with stands for angle tilting. In [66], which indicates different factors that can impact the stand-alone solar panel, the authors used online monitoring platforms to analyze the results for maximum power output tracking. This still requires further improvement for either the online platform or the images of the panel surfaces. From the research, installations of solar panels for rural or factory usage should be at a larger scale compared to household installations. Different types of PV installation are presented in Figure 9. Therefore, the need to have solar panels that can orientate and track the sun is greater for industrial uses than for rooftop solar panels for housing.

4.3. Dense Dust Layers

Having considered BIPV installation, the potential vertical distribution of atmospheric dust deposition can be considered. In [69], the authors indicate that dense dust layers, significantly affected by diurnal variations and boundary layer height, are more pronounced over the deserts and show similar patterns in dust aerosol optical depth, as observed by CATS and MERRA-2. However, this paper is based on the Taklimakan Desert, and considered the impact on the natural environment as well. However, [70] assessed particle transport performance by analyzing the distribution of the particle volume fraction within the computational domain. It also provided the result that building configurations characterized by a height-to-width ratio of 1 ($H/b=1$) demonstrated superior particulate dispersion efficiency. With a rise in building height, the transmission of particles was obstructed, resulting in lower transport efficiency. The use of a step-up building arrangement increased the airflow speed and facilitated the transportation of particulate matter. Conversely, the use of a step-down building led to a decrease in air flow speed and weakened the velocity fluctuation, minimizing the number of particles [71]. There is a lack of research on the dust suspension layer’s relationship to building height. Figure 10 presents the potential concept of the dust layer in the urban environment.

4.4. Safety Considerations in Urban PV Installations: Assessing Fire Hazards

It is crucial to comprehend the fire hazards related to dust accumulation on solar panels, especially in urban areas where these panels are becoming more commonly included in the architectural landscape. Solar panels, although being an environmentally beneficial energy option, are still impacted by the difficulties caused by urban soiling [73]. Urban soiling refers to the combination of dust, pollutants, and organic waste that is commonly found in city atmospheres. This soiling will affect the efficiency of solar energy conversion and cause significant fire risks.

As researched, the hazards are intensified in urban areas because of the proximity of buildings and the increased chance of various and possibly dangerous substances collecting on the surfaces of solar panels [74]. Moreover, the intricate interaction of urban environmental elements such as pollution, weather patterns, and human activity introduces several levels of complexity to the situation of fire danger. As urban areas increasingly embrace solar technology, it is crucial to mitigate these risks by conducting research, implementing appropriate design measures, and establishing policies that promote the secure and efficient utilization of solar panels within the framework of achieving a sustainable urban future.

By reviewing [75], some common causes of fire accidents in solar panels include installation errors, poor quality of PV modules, hot spot effects [76,77,78], and physical damages such as cracks. These factors can lead to cable faults, ageing components, and other conditions that increase the risk of fire accidents. In [75], the authors present experiments which demonstrated that shading conditions could lead to hot spots and significant shifts in the I-V curve, which affect power generation. In Figure 11 and Figure 12, real-life examples are presented.

Within the domain of photovoltaic (PV) system safety, fire scenarios are categorized into two main groups: originating fire scenarios and victim fire scenarios [82]. The primary source of original fire scenarios is typically internal to the system. These encompass both internal and exterior physical defects such as cell damage, panel cracks, and poor electrical connections. All these issues can lead to overheating and potentially trigger a fire.

Furthermore, environmental variables, such as the build-up of dust on panels and errors in shading, worsen the danger of fires by interrupting regular functioning and encouraging circumstances of excessive heat. Electrical malfunctions, including problems such as localized areas of high temperature caused by uneven solar radiation, inconsistencies within the panel arrangement, electrical sparks, and defects in the grounding and electrical connections, further increase the risk of fire events in photovoltaic (PV) systems [83]. Conversely, incidents of fire involving victims are impacted by external causes. Artificial events, such as fires beginning in nearby areas or buildings, have the potential to spread to the photovoltaic (PV) array, leading to the ignition of the system. In addition, photovoltaic (PV) systems are vulnerable to several natural dangers such as lightning strikes, typhoons, wildfires, and excessive heat, all of which have the potential to lead to the ignition of the system. Having a comprehensive understanding of both interior and exterior fire scenarios is essential for improving the safety measures and fire-prevention strategies of PV systems, especially in urban environments where the impacts of fire incidents are often more severe. A summary of potential risk factors associated with pollution in an urban environment is shown in

Table 4. Potential risk factors associated with pollution in an urban environment.

Risk Factor	Description	Mitigation Strategies
Flammable material accumulation	Accumulation of flammable debris like leaves or organic material on solar panels.	Regular cleaning of panels to remove flammable materials; installation of barriers to prevent debris accumulation.
Electrical arcing and hot spots	Uneven soiling leading to hot spots and potential electrical arcing.	Regular examination and cleaning to ensure uniform dirt accumulation; monitoring system health to promptly identify hot spots.
Reduced productivity and excessive heat generation	Soiling results in reduced efficiency, leading to the panels overheating.	Implementing regular cleaning schedules; installing cooling systems or heat-resistant components into solar panels.
Deficient inspection and maintenance	Soiling obstructs the ability for inspection, resulting in maintenance being neglected.	Implementing regular inspection and maintenance schedules; using technologies for remote monitoring and diagnostics.
Chemical reactions	Chemical components in soiling can react with the materials of the panel, potentially leading to deterioration.	Use of protective coatings on panels to prevent chemical reactions; regular testing to ensure material durability.
Environmental conditions	Varied urban environmental factors like pollution can enhance soiling risks.	Selection of solar panels designed for urban conditions; integrating environmental monitoring for a proactive response.

, providing a detailed overview of the description and mitigation strategies.

5. Regional Variations and Comparisons in Urban Soiling and PV Implementation

Urban soiling, a widespread problem in the field of environmental research, has significant variations in different countries. Investigating the impact of dust accumulation on the energy generation of large-scale PV plants in southern Italy, ref. [84] compares the performance of two 1 MWp solar parks both before and after cleaning their PV modules, considering the influence of soil type and washing technique on energy losses caused by pollution. The study used PV modules with a total nominal power of 0.99 MWp, equipped with a monitoring system to collect electrical and climate data for both soiled and cleaned modules. The regression model showed a strong correlation between the measured and predicted power, indicating the efficacy of the cleaning operation in improving the performance of the PV modules. This study found that the type of soil and washing technique significantly influenced the losses caused by pollution, with the plant experiencing a 6.9% loss on sandy soil, whereas the plant only experienced a 1.1% loss built on more compact soil.

Aside from Europe, the United States is emerging as a country with a growing number of photovoltaic (PV) plants. The authors of [85] calculated that soiling led to a decrease in power output on photovoltaic (PV) modules in a large-scale solar energy system in California. Information was gathered from two groups of inverters. One of the inverters was attached to 11 module strings and recorded the energy generation characteristics before the differential cleaning. The secondary inverter pair, linked to 12 sets of module strings, exhibited comparable energy generation patterns. One inverter and its related module strings were chosen as the control from each set, and they were scheduled for weekly cleaning. It was noted that the cleaned array constantly surpassed its counterpart in power generation.

Asia contains the other countries with the most solar plants installed. The authors of [33] aimed to find the solar energy output attributable to dust and anthropogenic particulate matter (PM) across regions like China, India, and the Arabian Peninsula. In this research, which utilized particulate sampling from solar panels coupled with General Circulation Model (GCM) simulations to evaluate the radiative forcing of ambient PM, they estimated the possible outcome or influence of settled PM on solar energy. Their findings indicate a pronounced diminution in solar energy yield, ranging from approximately 17% to 25% across the studied regions. Considering their extant solar generation capabilities, PM accounts for an estimated decline of around 1 GW in India and a significant 11 GW in China. This highlights the substantial impact of PM in reducing solar power production. The deposition of particulates appeared to be steered by dust and pollution incidents spanning several days to approximately a week [68]. However, this paper mainly considered particle impact, while the dust fall rate was not indicated.

In [86], the authors evaluated the dust fall rates and analyzed the mineral composition of airborne dust particles. The selection of atmospheric dust fall sample locations was based on land use plans and climatic conditions. Dust fall samples were collected according to the guidelines in the Indian standard procedures for measuring air pollution and the standard method for collection and analysis of dust fall. They also stated that the dust fall rates were influenced by variables such as humidity, wind speed, and temperature. The study found that the dust fall rates in mining areas were significantly greater than the standards set by various nations, including the standard level set by NEERI, Nagpur [68]. As global soil degradation is a critical environmental issue, Figure 13 indicates the higher rates of soil degradation with regions in greater green areas.

6. Discussion of the State of Art in Urban PV System Soiling Management and the Research Gap

6.1. Innovations in Soiling Management

The implementation of urban soil management has emerged as a viable method to address these challenges. In [30], the authors investigated soiling losses in PV systems in Hong Kong, which is a high-density urban environment, presenting a general idea of the main soiling factors and potential natural cleaning such as natural wind and rainfall which help reduce dust accumulation. The degradation of soil in urban environments presents complex challenges that require the use of interdisciplinary approaches to tackle and rehabilitate it. Moreover, the lack of regular maintenance leads to the gradual accumulation of dirt, worsening its harmful impact on the panels. Construction activities near solar systems can generate dust and particles that gather on solar panels, worsening their degradation. These aspects emphasize the significance of frequent cleaning and maintenance to reduce the negative effects of soil degradation on the effectiveness of solar panels. Cleaning methodologies for PV panels, and different types of urban soiling and forms of particles, can also affect the technologies being used for cleaning. The potential strategies can also be different in application, as the preventative measures may change their effectiveness. Rather than regular human-controlled cleaning, Figure 14 [88] presents an existing cleaning robot for soiled solar panels; however, it cannot work on a panel with too great a tilt angle.

To reach the aim of this paper, to assess the urban soiling impact, ref. [89] presents the soiling impact in an urban setting in Colorado. It also examined the influence of dew on urban PV soiling within a state presumed to have low soiling. A soiling station was constructed, equipped with an automated brush that activated daily in the morning, cleaning one of the reference cells. This station was strategically positioned at the CDPHE site to align with top-tier meteorological and air quality instruments. The findings underscore the notable soiling potential of urban areas, a concern set to amplify as urban expansion demands more power in growing cities. Within urban locations, where such contaminants may accumulate on PVs, it is a priority to emphasize wet contact cleaning methodologies to efficiently address this form of contamination.

The authors of [90] aimed to obtain the data on the loss of soiling from stations with unsatisfactory cleaning schedules and correct errors in the data caused by soiling of the clean cells [90]. This study considered ten soiling stations and recorded data in the southwestern United States. Each was equipped with two PV cells, one clean and one allowed to naturally soil. An algorithm was developed in this study to rectify errors in the observed soiling ratio caused by infrequent cleaning. The algorithm detected upward and downward shifts in the soiling ratio and interpreted them as cleaning events or soiling events, respectively. It also considered precipitation events and connected acceptable data points to determine the soiling ratio between them, with the recorded soiling ratios ranging from 93.6% to 99.9%. In [91], the results also suggested that the PV cells exposed to pollution had a reduction in power output of around 12% compared to the unpolluted cells. In contrast, ref. [92] focuses on the soiling problem in photovoltaic (PV) systems and its direct influence on energy production. It optimized cleaning schedules by collecting information about the soiling and considered change points (CPs) in the soiling extraction models. The authors of [93] examined the impacts of dust collection caused by dew on PV modules. An automated water-based cleaning technique was also suggested to reduce the negative effects of soiling on the electrical performance of solar PV modules. The experiments were performed at different tilt angles (23°, 33°, and 43°) to evaluate the effectiveness of the cleaning system. The power recovery after twenty days of efficient cleaning was measured at each tilt angle. The amount of water required for cleaning and the impact on the surface temperature were also recorded. The newly proposed cleaning system effectively removed dust, resulting in a power recovery of 12.07%, 11.40%, and 9.92% at the respective tilt angles. The cleaning method required an average of 7.325 L of water per 1.48 m² of the solar PV module. The cleaning method resulted in a decrease in the front and back surface temperatures of the PV modules, providing a cooling effect.

6.2. Research Gap

Upcoming technologies addressing urban soiling on PV panels have the potential for integrating urban solar power plants with other urban infrastructures. Having reviews of the technologies,

Table 5. Potential research gaps of urban soiling on solar panel technology.

Research Gap	Description
Composition and characteristics of urban soiling	Research is required to comprehend the specific composition of urban soiling, especially in various urban environments, and the effect on solar panel performance.
Impact of diverse urban pollutants	Studies focusing on how specific urban pollutants like vehicle emissions and industrial byproducts impact the efficiency of solar panels are limited.
Long-term degradation studies	There is a lack of longitudinal studies on how consistent exposure to urban soiling affects the structural integrity and performance of solar panels over time.
Effective cleaning and maintenance Strategies	Research into cost-effective, efficient, and sustainable methods for cleaning and maintaining solar panels in urban settings is lacking.
Soiling prediction models	Development of accurate and localized prediction models for soiling accumulation that consider the urban-specific factor is lacking research.

presents the potential research gaps of the reviews in this paper. Reviewing these papers, it states that further research and improvement are needed in cleaning and maintenance. As mentioned in the past reviews, different particles and urban pollutants are considered without discussing their potential differences in the cleaning aspect. The other significant research gap in the past reviews is the interaction between urban microclimates and BIPV performance, which is an under-researched area. Urban heat islands, local wind patterns, and other microclimatic factors could significantly influence soiling and solar efficiency.

7. Conclusions

Solar photovoltaic (PV) systems are severely impacted by urban soiling, a significant problem in densely populated regions. This phenomenon encompasses the build-up of several types of pollutants on solar panels, including dust, automobile fumes, and industrial waste.

Due to regional variations in local environmental, industrial, and climatic factors, urban soiling composition differs greatly between various worldwide locations. Interestingly, reviews indicate that vehicle emissions and industrial emissions are the main causes of urban soiling in rapidly industrializing countries like China and India. In contrast, because of differences in urban and industrial activity, different regions in the USA and South Africa have different soiling profiles. Maintaining solar panel efficiency and maximizing cleaning techniques require an understanding of these geographical variations.

The variety observed in an area is mostly shaped by its unique urban attributes, such as the industrial activity, traffic intensity, meteorological circumstances, and existing land-use patterns. In industrialized cities, soiling mostly comprises particle emissions originating from manufacturing activities, but in urban centers with high traffic, automobile emissions have a notable role as a contributor. Humidity and rainfall are important elements that significantly influence the nature and buildup of urban soil.

In this paper, a review of urban soiling, the general idea of further cleaning and possible mitigation strategies is essential. The potential technologies of solar systems in urban areas, for example, BIPVs, have been considered in this paper as case studies to understand the most possible direct causes of urban soiling in the urban environment.